People using the reason that the SHs never kill to oppose the idea of Zoro killing Kaidou is the same person who argued that Zoro won't fight Kaidou because only Luffy can fight the Big Bad Boss
As far as i'm concerned, Zoro, Luffy, or anyone in the SHs never stated that they won't kill anyone
The reason they never killed because Oda wanted to save the villians for future plots.